The mysterious dark storm was discovered by Hubble during Neptune's southern summer.

NASA's Hubble Space Telescope has found strange storms that are raging on the surfaces of Neptune and Uranus, which could help solve the mystery of the storm circling around Uranus' north pole.

Much like Earth, the gas giants Uranus and Neptune also have seasons that probably drive some of the features and seasonal changes in their atmospheres. However, their seasons are much longer than on our planet. One summer on Uranus, for instance, would last 42 Earth years.

The storm first appeared during the planet's southern summer, the fourth and latest mysterious dark vortex captured by Hubble since 1993.

Two other dark storms were discovered by the Voyager 2 spacecraft in 1989 as it flew by the remote planet, NASA said.

Since then, only Hubble has had the sensitivity in blue light to track these elusive features, which have appeared and faded quickly.

A study led by Andrew Hsu, an undergraduate student at the University of California, Berkeley in the US, estimated that the dark spots appear every four to six years at different latitudes and disappear after about two years.

Hubble uncovered the latest storm in September last year in Neptune's northern hemisphere. The feature is roughly 6,800 miles across.

To the right of the dark feature are bright white "companion clouds." Hubble has observed similar clouds accompanying previous vortices.

Like Jupiter's Great Red Spot, the dark vortices swirl in an anti-cyclonic direction and seem to dredge up material from deeper levels in the ice giant's atmosphere.

The Hubble observations show that as early as 2016, increased cloud activity in the region preceded the vortex's appearance.